In my opinion, your Hebrew level should be at a minimum of a level daled ulpan before you start studying for the exam because the exam requires advanced reading comprehension and written answers in Hebrew. At this level, I believe you maintain the necessary skills needed to learn and understand the legal Hebrew terminology, as well as to understand the questions you will face on the exam. Once you learn the legal concepts and definitions (and case examples), you are on your way to easily passing the Hebrew exam! Further, if you are at a minimum level of daled, studying for the exam approximately three weeks prior to the exam should be sufficient (although all depends on each one's personal study methods and the pace of one's learning, which of course varies from person to person). Although, I don't think it's impossible to pass the exam if you have a weaker grasp of the Hebrew language. However, it would definitely be more challenging and I'd recommend in that case a more rigorous course of study.
For assistance in learning the required materials and for study techniques to beat the exam, please feel free to contact me.